Publications (1)0 Total impact
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ABSTRACT: (abridged) Non-ionizing stellar continua are a source of photons for
continuum pumping in the hydrogen Lyman transitions. In the environments where
these transitions are optically thick, deexcitation occurs through higher
series lines, so that the flux in these lines has a fluorescent contribution in
addition to recombination; in particular, Balmer emissivities are
systematically enhanced above case B. The effectiveness of such mechanism in
HII regions and the adequacy of photoionization models as a tool to study it
are the two main focuses of this work. We find that photoionization models of H
II regions illuminated by low-resolution population synthesis models
significantly overpredict the fluorescent contribution to the Balmer lines.
Conversely, photoionization models in which the non-ionizing part of the
continuum is omitted or is not transferred underpredict the fluorescent
contribution to the Balmer lines, producing a bias of similar amplitude in the
opposite direction. In this paper, we carry out realistic estimations of the
fluorescent Balmer intensity and discuss the variations to be expected as the
simulated observational setup and the stellar population's parameters are
varied. In all the cases explored, we find that fluorescent excitation provides
a significant contribution. We also show that differential fluorescent
enhancement may produce line-of-sight differences in the Balmer decrement,
mimicking interstellar extinction. Fluorescent excitation emerges from our
study as a small but important mechanism for the enhancement of Balmer lines,
which should be taken into account in the abundance analysis of photoionized
regions, particularly in the case of high-precision applications such as the
determination of primordial helium.
10/2008;
